In the past, nitrous oxide (N.sub.2 O) has received little attention as an atmospheric pollutant and has not been considered a constituent of the gaseous pollutants collectively referred to as nitrogen oxides (NO.sub.x) which have received wide attention as pollutants harmful to the environment. However, recent studies indicate that N.sub.2 O in the Earth's atmosphere may be increasing by about 0.2% per year and that this increase appears to be caused by anthropogenic activity.
N.sub.2 O is a major stratospheric source of NO, has been linked to the destruction of the ozone layer and is recognized to be a green-house gas. Because N.sub.2 O has an atmospheric lifetime of approximately 150 years, a strong effort is underway to identify sources of the pollutant and to limit further production of the harmful gas. Recent reports such as an article by Thiemens and Trogler, Science, 251(1991)932 suggest that various industrial processes significantly contribute to the increased levels of N.sub.2 O found in the Earth's atmosphere.
For example, nitrous oxide is a by-product in the manufacture of monomers for producing 6,6- and 6,12-nylon. Approximately 1.24.times.10.sup.9 kg of nylon were produced in the United State in 1898, alone. Nylon polymers are typically formed by condensation polymerization of a dicarboxylic acid and a diamine. The most widely used dicarboxylic acid, adipic acid, is prepared primarily by air oxidation of cyclohexane to form a cyclohexanol/cyclohexanone mixture followed by oxidation with HNO.sub.3 to form adipic acid and N.sub.2 O.
According to calculations by Thiemens and Trogler, the reaction stoichiometry for producing N.sub.2 O in the preparation of adipic acid is about 1 mol of N.sub.2 O per mole of adipic acid. Assuming a global yearly adipic acid production of 2.2.times.10.sup.9 kg yr.sup.-1, about 1.5.times.10.sup.10 mol yr.sup.-1 of N.sub.2 O by-product or 10% of the annual atmospheric N.sub.2 O increase can be attributed to this single process.
According to the NOx information Book, March, 1983, DOE/NBB-0044, prepared for the U.S. Department of Energy, N.sub.2 O was not previously considered a major atmospheric pollutant. For example, Chapter 2, "Characteristics and Origins of Oxides of Nitrogen", states that the term `NOx` is used in the air quality field to signify NO and NO.sub.2. The reference also states that "N.sub.2 O is not considered an air pollutant, but it is a principal reactant in upper atmospheric reactions decomposing ozone". This preliminary data may explain why N.sub.2 O emissions are not currently regulated by the Environmental Protection Agency.
Several catalysts have been studied for N.sub.2 O decomposition. However, the objective of most of the literature studies was to develop an understanding of catalytic mechanisms. Slinkin and coworkers, Kinetika i Kataliz, 19(1978)992 and 20(1979)515, studied the catalytic decomposition of N.sub.2 O over dealuminized H-mordenites and mordenites containing nickel cations. The investigators reported reaction rates of 1.5.times.10.sup.-5 mol g.sup.-1 h.sup.-1 for H-mordenite and 9.5.times.10.sup.-6 mol g.sup.-1 h.sup.-1 for Ni-mordenite at 450.degree. C.
Hall and coworkers, J. Catal., 86(1984)392, studied the catalytic decomposition of N.sub.2 O over various iron-exchanged catalysts. Iron-exchanged Y zeolite provided a rate of 5.5.times.10.sup.-6 mol g.sup.-1 h.sup.-1 at 498.degree. C., and iron-exchanged mordenite provided a rate of 2.6.times.10.sup.-4 mol g.sup.-1 h.sup.-1 at the same temperature. Neither Na-M nor Na-Y performed as catalysts in the subject reaction.
Pomonis and coworkers, J. Chem. Soc., Farady Tran., 81(1985)2043, studied Fe.sub.2 O.sub.3 /Al.sub.2 O.sub.3 as a catalyst for N.sub.2 O decomposition. At 500.degree. C. and a space velocity of about 3,000 h.sup.-1, a conversion of less than 1% N.sub.2 O was obtained which is equivalent to a reaction rate of 1.4.times.10.sup.-5 mol g.sup.-1 h.sup.-1 at [N.sub.2 O] of 1000 ppm.
Dumesic and coworkers, J. Catal., 110(1988)330, studied N.sub.2 O decomposition over cation-exchanged Y zeolites (Fe--Y, FeEu--Y, Eu--Y, FeLa--Y, La--Y, Cu--Y, Co--Y, Cr--Y, Ni--Y and Mn--Y). Among these catalysts, FeEu--Y and Cu--Y were most active. At 450.degree. C., the reaction rates were 1.8.times.10.sup.-4 and 1.4.times.10.sup.-4 mol g.sup.-1 h.sup.-1, for FeEu--Y and Cu--Y, respectively.
Panov and coworkers, J. Mol. Catal., 61(1990)85, recently reported that FeZSM-5 was much more active than Fe.sub.2 O.sub.3, Fe--Y and Fe-mordenite in the decomposition of N.sub.2 O to gaseous nitrogen and oxygen. Comparison of these rates with the steady-state reaction rates of others is difficult because the investigators utilized a static system for their reaction.
Since N.sub.2 O is now considered an environmental pollutant due to its contribution to the greenhouse effect and to its catalytic effect on the destruction of the stratospheric ozone layer, an urgent need exists to develop catalytic processes for destroying N.sub.2 O emissions prior to venting commercial process effluent streams into the atmosphere. Although catalytic decomposition of N.sub.2 O has been studied extensively in academic institutions, no commercially viable process is known for decomposing N.sub.2 O into its respective components, namely gaseous nitrogen and gaseous oxygen.